1. Field of the Invention
The present invention relates to an optical element as a lens having a coated surface. The present invention also relates to an optical pickup device equipped with the optical element.
2. Description of Related Art
Conventionally, an optical pickup device is equipped with an objective lens (optical element) that condenses light from a laser diode onto an optical disc. Such an objective lens is made of a plastic material, a glass material or the like.
A molded objective lens may cause various aberrations (wave aberration) in outgoing light due to various factors, for example, tolerances of a lens surface or nonuniform distribution of the refractive index inside the lens. The various aberrations make a shape of a light spot formed on the optical disc different from a desired shape. Therefore, a shape error may occur between the deformed light spot and the desired light spot, which causes a phenomenon that data cannot be recorded on the optical disc stably, a phenomenon that data to be read from the optical disc cannot be reproduced correctly or other phenomena. Note that an objective lens corresponding to a blue color laser (having a wavelength of approximately 405 nm) to be with high accuracy is required to have the wave aberration below 10 mλ rms, for example.
One of factors that causes nonuniform distribution of the refractive index inside the lens is glass molding. When an objective lens OL is manufactured by the glass molding, base material of melted glass GM is molded by pressure with a die MM (MM1 and MM2) having predetermined curved surfaces as shown in FIG. 4. Therefore, relatively large pressure is exerted on the outer edge of the objective lens OL, and the pressure causes stress strain inside the objective lens OL. As a result, birefringence occurs (the number of arrows in FIG. 4 shows pressure distribution). This birefringence is likely to occur in a lens having a large numerical aperture (e.g., numerical aperture of 0.6 or larger) with a large thickness difference between the middle portion and the outer edge portion of the lens.
As one method for preventing the phenomenon as described above, there is a method as described in US2005/210922A1, for example. According to this method, initial design value of the optical element is determined first based on a precondition that refractive index distribution inside it is uniform. Next, the optical element is modeled based on the initial design value, and refractive index distribution of the molded item (initial item) is measured.
Next, various aberrations due to the measured refractive index distribution (i.e., nonuniform refractive index distribution data) is determined by simulation. Then, aspheric surface shape data that enables correction of aberration in this simulation is determined, so that the die is corrected and processed to match the aspheric surface shape data. Then, the die after the correction and process is used for molding, so that aberration of the optical element can be reduced.
However, the optical element disclosed in US2005/210922A1 requires very difficult and burdensome correction and process of the die. In addition, such an optical element requires burdensome measurement of nonuniform refractive index distribution data.